gdb: remove unnecessary parameter wait_ptid from do_target_wait

do_target_wait has a wait_ptid parameter, to filter what ptid we wait
on.  The sole caller of do_target_wait passes minus_one_ptid, meaning
"all ptids".  So in practice, this parameter is not needed, remove it.

gdb/ChangeLog:

	* infrun.c (do_target_wait): Remove wait_ptid parameter.
	(fetch_inferior_event): Adjust.

Change-Id: I54119beb43db678e4b2081dc490f89e7ff878e74